This is just a guess and I haven't run the numbers to see if it makes sense, but I would go with the external gauge. What you really want to know is the difference between atmospheric and internal pressure and your external gauge will show you that. If the TPMs where mounted at see level, they are probably calibrated against that atmospheric pressure and thus read a lower differential pressure. I'd be curious to find out how the TPMs are really calibrated. 8800 ft is pretty extreme, probably way outside the expected usage range.stanwagon-
I am having the same tire pressure issue. I am at 8800ft. I have several tire pressure gauges, they all show 39psi. The app shows 34. Maximum tire pressure is 44 on the stock tires. I am thinking I will be at or above the max on my tire gauge, to get the app at 38. For now I am going with my tire pressure gauge. I do not know the correct answer.
Sea level pressure is equal to 14.7 psi. 10,000 feet above sea level the atmospheric pressure is 10.1 psi. Thus there's a 4.6 psi differential between the two. The internal tire sensor can't reliably measure gauge pressure being inside the tire. Your external pressure gauge always measures the difference between the internal tire pressure and the atmospheric pressure. That's why they call it gauge pressure.But the huge discrepancies above are annoying to a numbers guy.
Yes. I am using my favorite bicycle sliding plunger gauge to set the Bolt's tire pressure. I have an air compressor with an analog gauge on the tank, and another at the air chuck. They don't match. I have four bicycle pumps. They don't match. And the TPMS don't match each other when cold, but DO match when warmed up after driving awhile. I don't find this at all surprising. I worked as a tech for some years, and finding two lab grade thermometers that gave the same reading, out of six, was a miracle. Calibrating instruments, and whole labs to give matching results is a nightmare. I often laugh about people quibbling over advertised Cd for cars, run at different times, in different wind tunnels. Unless they hauled a standard model from lab to lab, and calibrated to that standard, before every run, the differences they argue about probably noise.I am actually at a total loss as to how to know the pressure in a tire. .
Tastes differ, but I certainly would never run a tire over the manufacturer recommended max.I overinflate by about 10 PSI over factory to increase performance and economy at the cost of reduced ride comfort and increased noise.
I never trust the gauges on inflation devices, including the electric pump that came with the Bolt and the one on my good ol'-fashioned manual tire pump. They always seem to read high.
Exactly, which is why I preface my comments saying the matter is one of preference. I weigh the benefits of lower fuel consumption and better performance with the drawbacks of harsher ride, increased noise, and increased suspension loads when determining how much to overinflate.Tastes differ, but I certainly would never run a tire over the manufacturer recommended max.
Note that once your tire warms up, you would be at about 6 lbs. over the 45lb. max.
Oh, and BTW, 10 lb. overinflation will also shake the devil out of the car from the harder impacts, and will make the interior noisier sooner.
Actually, the software doesn't do anything but read back the pressure. The original calibration was done with sea level as its reference (they used a gauge). The sensor is only capable of reading the absolute pressure (because it is closed inside a tire). But the software shows the pressure as gauge which assumes the sea level correction (or possibly some other reference as a compromise). The software needs to do more than just have a fixed calibration curve that is only adjusted for sea level. If they had calibrated the device to report back in absolute pressure, then gauge pressure can always be calculated and displayed correctly by knowing the local atmospheric pressure (ambient). Instead, it shows the pressure that only reads accurate at sea level.(b/c of software and subtraction of sea level ambience).
Actually, the pressure does drop slightly because the tire will grow without the added external pressure. The ideal gas equation is PV=nRT. The number of molecules of air is the same. Assuming the same temperature. The pressure will drop because the volume of the tire will increase. So your physics friend might not be fooled. Although the drop in absolute pressure is not very much.Now: If I ask a physics colleague: what happens to the measured tire pressure on going from 0 to 10000 ft: it is a trick question, since the method of measuring is not specified in the question.
Apologies Stanley. I was inspired by the photo to Google the Stanley Steemer and realized I had misspelled your name, but forgot to edit my post.It's Stanley. So my name is identical to that old car whose pic I show: A "Stanley Wagon".
Years ago I learned that barometric altimeters are calibrated to the yearly average b/c of temp. fluctuations